Heat the oil in a skillet over medium heat. Add the onion and zucchini and sauté until golden. Add the beaten eggs but do not mix. Allow eggs to set. Sprinkle with shredded cheese. Lower heat and cook until rim of frittata begins to turn brown. If your skillet has a plastic handle, cover with silver foil to avoid melting. Place skillet with frittata into a broiler and broil on high for 2-3 minutes. Tip: If you do not have a milchig broiler or oven available on Pesach, leave the skillet over medium heat for a few more minutes until eggs seem to have set completely.